There are numerous situations in which it is necessary to supply fluids in small, very precise flow rates. For example, in agriculture it is common to utilize irrigation systems in which relatively small amounts of liquid fertilizers, pesticides or herbicides are added to the water. Often these chemicals are added at volumetric flow rates as little as one-half gallon per hour, whereas the volumetric flow rate of the irrigation water may be several hundred gallons per minute. Also, in the production of pharmaceutical products, the active ingredient may compose a fraction of one percent of the total weight or volume of the product. It is highly important that the proportion of the active ingredient be very accurately metered during the production of the pharmaceutical. As another example, in food processing certain ingredients, such as dyes, fixatives, preservatives or spices comprise a small fraction of the total food product. Again, in these situations it is critical that these particular ingredients be very accurately metered during the food production process.
In one typical type of system commonly used in agriculture, an electric or internal combustion motor is coupled to the input shaft of a separate, speed-reducing gearbox. A variable throw crank is mounted on the output shaft of the gearbox which is disposed at 90.degree. from the input shaft. A connecting rod interconnects the crank with an elongated piston to reciprocate the distal or free end of a piston within a pumping chamber. Check valves are placed in the inlet and outlet ports of the pumping chamber ostensibly to prevent the liquid in the pumping chamber from leaking back through the inlet port as the piston is advancing into the pumping chamber to force the liquid out through the outlet port and, conversely, to prevent the liquid from leaking back into the pumping chamber from the outlet port when the piston is being retracted to draw liquid into the pumping chamber through the inlet port. This type of pumping system suffers some significant drawbacks. These systems are typically composed of a menagerie of "off-the-shelf" components which are not well matched in geometric configuration nor relative size or capacities. As such, it is necessary to "oversize" many of these components, resulting in not only a larger, heavier and more expensive system than actually required for the desired function, but also requiring a high level of energy consumption relative to the volume and pressure of the liquid being pumped. In addition, the corrosive nature of the liquids being pumped often causes premature failure of the pump mechanism. Also, the system typically requires frequent lubrication and maintenance, which may not always be performed in the agricultural setting. Further, the mechanism for changing the stoke of the crankshaft typically cannot be adjusted in a precise manner so that often the fertilizer, pesticide or herbicide is applied at either too high or too low of a rate.
Because of their low volumetric flow rates, diaphragm-type pumps also are utilized in irrigation, food processing and pharmaceutical production to supply liquid ingredients at small flow rates. In one type of diaphragm pump, the diaphragm is flexed back and forth by a reciprocating plunger having its forward end secured to the center of the diaphragm. Examples of such diaphragm pumps are disclosed by U.S. Pat. Nos. 3,288,071 and 4,368,010. These types of diaphragm pumps suffer from several serious drawbacks. For instance, the mechanism for varying the flow rate of the pumps often is not finely adjustable enough to control the supply of liquids as accurately as required. Also, in many known diaphragm pump designs, if the diaphragm ruptures, the liquid being pumped mixes with the pump lubricant located on the other side of the diaphragm and thus becomes contaminated. A further common drawback of known diaphragm pumps is that during normal operation the components of the pump are subjected to relatively high stress loads resulting in failures or unreliable operation after a relatively short time period.
In a second known type of diaphragm pump, the diaphragm is actuated by hydraulic fluid which pushes against the side of the diaphragm opposite the liquid being pumped. The hydraulic fluid is cyclically pressurized by a reciprocating piston. In addition to the shortcomings of plunger-actuated diaphragm pumps discussed above, in hydraulically powered diaphragm pumps, if the diaphragm leaks or ruptures, the higher pressure of the hydraulic driving fluid causes the fluid to be injected into the liquid being pumped, thus contaminating the liquid. Further, the cyclical pressurizing of the hydraulic driving fluid results in the generation of large amounts of heat causing the temperature of the fluid to rise to high levels, often leading to premature failure of the pump, including the diaphragm, which typically is one of the more "fragile" components of a diaphragm pump.